Project Summary/Abstract Inflammation is is a highly complex process that is a component of most forms of pulmonary disease. It represents a response to tissue insult and normally (ideally) leads to return of tissue homeostasis, although, depending on the initating stimulus, the repair process may also involve a transient fibrotic response (akin to ?wound heaing? in the skin). Macrophages are essential cells in these events, playing key roles in all steps in the processes, orchestrating many elements of the repair/resolution and return to homeostasis; the understaning of which is the primary focus of this proposal. Macropahges in the lung fall into two general categories: 1) resident macrophages involved in maintaining the homeostasis and serving as sentinels to detect the initial stimulus or injury and 2) recruited macrophages maturing from incoming monocytes. We and others have recently identified unique macrophage subtypes (resident and recruited) within the pulmonary interstitium in addition to those in the airspaces, but at this point, their specific responses to, or participation in, the inflammatory responses and their resolution have not been determined. Accordingly, during this project we will elucidate the roles for individual macrophage subsets in the resolution phase of the inflammatory response, with the ultimate objective of devising therapeutic approaches for its enhancement. Using novel approaches to target, lineage trace and manipulate the different macrophage populations we will determine their numbers, functional programing state, and critically their specific location within the lungs over the course of inflammation induced by four unrelated stimuli ? two that lead to early resolution (bacterial lipopolysaccharide and H1N1 influenza infection) and two that include a transient fibrotic response during a more prolonged resolution (bleomycin or HCl, the latter mimicking exposure to gastic contents). During inflammation, the resident macrophages within the normal lung interstitium and airspaces increase in numbers but also are joined by large numbers of recruited macrophages. In the processes of resolution, these excess macrophages are removed and a major focus of the proposal will be determination of the modes of removal and the effects of deliberately enhancing (or delaying) this removal on the desired return to homeostatic lung structure and function. Key hypotheses to be explored include: 1) Unique macrophage properties during both iniation of inflammation and especially its resolution, for subsets of interstitial macrophages (vs. those in the airspaces) relating to their precise localization, for example within the bronchovascular bundles or in the subpleural region). 2) Macrophages are recruited to inflamed lungs in waves, exhibiting unique programing properties, with later recruited cells playing the key roles in inflammation resolution, including both the induction and resolution of the transient fibrotic response. 3) The recruited macrophages themselves, in both the transient and extended inflammatory circumstances are removed after undergoing extrinsic apoptosis.